Cross Section Drawings are something that was available all over the books two decades /or more/ ago. Especially Russian ones and Soviet publications before them. Cross section drawings depicted different cutaways of the item on the blueprints. For example, with airplanes, usually nose cone, behind the cockpit, before and after the wings and tail section featured cross section. Those were marked with letters from the alphabet on the side view of the blueprint, and then without a legend – shown as a form right next to it.
No interior features were presented. Only an area outlined in a darker shade of the same color, just to grasp the idea about the plane’s /or other vehicle/ general form. Having those gave the opportunity to easily comprehend the actual form of the object. Back in a day, pictures were done on a camera film, thus it was an expensive endeavor to have multiple pictures and to pick from them. That created problems with the depth perception of the viewer and 3D understanding of the object presented. Mainly due to the lack of photos. Sometimes, pictures were deliberately made from weird angles, due to secrecy concerns, which with the Eastern Block was a maniacal devotion during the Cold War. They loved picturing jets from the front and below. You can hardly get anything to work with.
Nowadays, we have digital photography. We can get tons of pictures, videos, 3D drawings and what not. Objects cannot remain hidden once seen in public, because almost everybody has a tool to take picture or video with. That made the cross section drawings obsolete to some extent. At least for books, magazines or similar publications. They remained in the past. From several pictures and different angles, one can now easily create a mental picture about the object and with the help of software, even get the accurate measurements of it.
However, cross section drawings are still present when blueprints of an RC or an exact replica is to be made. Usually they are not needed when we are talking about the original product, because assembling everything should eventually give the exact shape without any further explanation needed. But yes, they are still there, even though scarce. Some old-school magazines still present us with drawings featuring cross sections. Sometimes even more, when it comes down to weird designs like Douglas X-3 Stiletto. Or some rockets. Maybe with airliners sitting layout too. Then, there is no other way but feature those. But in general, for regular user out there, cross sections are like dinosaurs – almost extinct.